An inverter is an electrical device enabling generation of a voltage with a variable frequency. One example of an inverter is a converter bridge implemented by IGB transistors (IGBT, Insulated-Gate Bipolar Transistor) or other power semiconductor switches. Inverters may be used in connection with photovoltaic (PV) applications where the inverter converts the DC voltage produced by one or more photovoltaic panels to an AC voltage and feeds it to an AC supply network; e.g., the inverter generates an AC voltage with a frequency that corresponds to the frequency of the AC supply network. Such an inverter may be called a solar inverter.
A solar inverter may start the DC-to-AC conversion of power supplied by the photovoltaic panel(s) on the basis of an open-circuit voltage supplied by the photovoltaic panel(s). The term open-circuit voltage refers to a voltage between the two output terminals of the photovoltaic panel(s) when there is no external load connected to the output terminals and thus when essentially no current flows through the output terminals. The conversion operation in the inverter may start if the determined open-circuit voltage exceeds a set reference value and after a possible predetermined delay set for the start.
The above way of starting the DC-to-AC conversion does not necessarily enable an accurate determination of an optimal starting moment because the open-circuit voltage of a photovoltaic panel may reach its maximum value at such a low level of current output capacity that it is not feasible to start the DC-to-AC conversion. By using starting delays it is possible to try to better adjust the starting moment to an optimal one but because of changing conditions, such as temperature or snow load on top of the photovoltaic panel, it may be difficult to keep the starting moment optimal.
For example, it is possible that there is some snow on top of the photovoltaic panel such that the open-circuit voltage of the photovoltaic panel does reach a sufficient level to trigger the start of the DC-to-AC conversion but still the current output capacity of the photovoltaic panel is not yet sufficient. As a result, the solar inverter may unnecessarily keep starting and stopping the DC-to-AC conversion, which wastes energy and may be detrimental to the solar inverter in the long run.